1. Technical Field
The present invention relates to optical switches for use with fiber-optic cable waveguides. In particular, it provides a non-mechanical electro-optic 2.times.2 crossbar-type switch which can be expanded to a 2.times.N or 1.times.N switch.
2. Background
Communication using guided light waves has become increasingly practical as fiber-optic cables have improved. For use with 1.5 .mu.m and shorter wavelengths, these are constructed from silica glass as a small diameter, 5-100 .mu.m core, surrounded by a higher index of refraction cladding layer. The cable acts as a circular waveguide. If the core is as small as a few wavelengths, e.g., 8-10 .mu.m, only one mode of light can be transmitted. These means that all the light will be in phase and travel coherently for long distances. Currently, single mode fibers are commercially available capable of transmitting 1.3 .mu.m and 1.5 .mu.m wavelength light with losses below 0.5 db/km. The glass part of these cables is only 125 .mu.m in diameter so that many fibers can fit in a small space.
In addition to fiber-optic cables, a number of other components are required for optical communications systems. These include laser diode light sources, photodetectors, permanent or temporary fiber joining devices (splices and couplers), amplitude modulators, optical isolators, and switches. Many of these components have acceptable performance characteristics, but available switches are not satisfactory for many applications.
One class of switches uses fibers which are cut precisely and held in fixtures. Switching is accomplished by butting the end of a selected fiber to another using solenoids, motors and the like. A number of ingenious mechanisms have been developed which can maintain close tolerances on the positioning of fibers. Fiber-to-fiber losses as low as 0.1 db can be achieved even for switches with a large number of incoming and outgoing fibers. Other approaches involve moving mirrors. However, being mechanical devices, the speed at which switching can be accomplished is in the 10-100 millisecond range. Another type of switch uses liquid crystals which can switch no faster than about 100 microseconds. They are generally designed to handle a small number, e.g., two input fibers and two output fibers, but if the losses are low enough this is an acceptable approach. The telephone industry has long used the 2.times.2 crossbar switch to build more complicated switching configurations. In this switch, each of two inputs are connected to two outputs, physically directly across from the inputs (the bar state), or the connections are crossed over (the cross state).
A very high speed crossbar switch with nanosecond switching speed can be constructed on a lithium niobate substrate. In this type, the fibers are coupled to quasi-rectangular thin film titanium waveguides diffused into the lithium niobate. These are arranged to be within a few micrometers of each other. Ordinarily, the light from the two waveguides does not interact and travels straight through. However, if an electric field is applied to the lithium niobate, its index of refraction will change and a condition can be set up where the light in each of the two waveguides crosses over to the other. The major drawback of the lithium niobate switch is that the losses are fairly high. It is difficult to couple light from the round fiber core waveguide into the rectangular waveguide on the substrate. In addition, this switch only works for singly polarized light.
A major problem faced in manufacturing fiber-optic components is alignment of fibers. When two fibers are to be mated to each other, precision mounting hardware is often adequate. However, when there are intervening optical elements with even slightly distorted optical paths, optical alignment requires injecting light in one fiber and moving the second and possibly additional fibers until maximum transmission is achieved. At this point the fibers are secured with UV-curing adhesives. This is tedious because, for 10 .mu.m-core single-mode fibers, the alignment must be withing 1 .mu.m radially and 1.degree. in angle for the insertion loss to be less than 0.1 db. A faster alignment method would greatly reduce the cost of packaging these devices.